There is often a need to externally specify a constant to a digital electronic system. In particular, empirically determined parameters or user modifiable features used in specific systems often employ externally specified constants. These constants are employed to modify the operation of the system in particular ways.
There are two main methods used in the prior art to provide an external specification of constants to a digital electronic system. The first of these involves a set of user activated switches. These switches could be used manipulated to indicate a set of one or more binary digits. In accordance with the prior art these switches are often provided in dual in line packages which may be mounted on circuit boards in the same manner as integrated circuits. This packaging technique makes such switches easy to incorporate into a digital electronic system. In addition, these sets of switches are relatively inexpensive. There are disadvantageous features to this technique. Firstly, it is often very difficult for the user to manipulate these switches. Particularly if a number of such switches are mounted in a dual in line package, they are very small. A small instrument, such as a pencil point, is often needed to manipulate these switches. The number of connections required to couple such switches to the digital electronic system is directly proportional to the number of bits required. Many digital electronic systems, particularly low cost microprocessor systems, have only a limited number of connections for all interfacing and thus do not have many connections to devote to such sets of switches.
A second technique employed in the prior art involves the use of a read only memory or a programmable read only memory. These memories can store large amounts of digital data, and thus can be used to provide external specification of a number of constants. These memories differ chiefly in their manner of programming. Read only memories have the data fixed during their manufacture and cannot be changed once constructed. Programmable read only memories are constructed capable of accepting later programming. Some types of programmable read only memories are erasable and capable of being reprogrammed while other types are not. These memories are generally used in the same manner. They are connected to the digital electronic system to an address bus, which enables the digital electronic system to specify which data to be read, and a data bus, which permits the data to be transmitted from the memory to the digital electronic system. This type of interface is more complex than that described above and is more expensive to construct. Because of this and because such memories are capable of specifying a large amount of data, such a system is generally economical only when a small amount of data is to be specified.
It is therefore a need in the industry to provide a low cost method of specifying an external constant of a limited number of bits which does not require a large number of connections to the digital electronic system.